Hunting For Habitable Planets
January 31, 2013

New Model To Help Identify Habitable Zones Around Stars

John P. Millis, Ph.D. for — Your Universe Online

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The holy grail of planetary astronomy is the elusive Earth-like planet that scientists hope will show that life exists elsewhere in our galaxy. With a growing database of more than 2,000 planet candidates, researchers feel it is only a matter of time before a planet is found with the potential to sustain life.

This gives rise to an interesting question: How will we know if a planet can truly support life?

While the specific circumstances under which life arises is still a mystery, current theory suggests that the presence of liquid water and a stable atmosphere are crucial elements. The latter means that a planet cannot be too small, otherwise the gravitational pull of the planet would be insufficient to keep the necessary gases from escaping into space.

The question of water, however, is another matter. It will be challenging to directly determine if water is present on the surface of an extrasolar world, but we do know that certain conditions would have to be met for liquid water to even be a possibility.

In addition to the planet´s size and composition (gas giants, for instance would not maintain oceans of water) the main contributing factor is the planet´s surface temperature. We can estimate the range of temperatures from two factors: the size of the planet´s host star, as well as the orbital distance between the two.

Of course other factors, like the composition of the planet's atmosphere, the presence of clouds and atmospheric density, all play a role in determining the surface conditions. A classic example is Venus, where the harsh dense atmosphere causes a runaway greenhouse effect. The result is a surface temperature hot enough to melt lead.

This is why new models, like the one soon to be published in The Astrophysical Journal created by researchers from the Penn State Department of Geosciences, are changing the way we think about habitable zones — the regions around stars where liquid water is possible. Taking into account more advanced absorption models, researchers are finding these zones are farther away from the host stars than previously anticipated. In fact, it seems that our own planet may actually be close to the inner boundary of the solar system´s habitable zone.

Ultimately, such studies will inform how and where we look for Earth-like planets. And when we find one that is just the right size and right distance from its star, the search will be on to find signatures of life on the surface.

An interactive calculator to estimate Habitable Zones is available online.